1. Field of the Invention
The present invention is generally related to a frequency counter and a method thereof, in particular to the frequency counter and method adapted to a handwriting apparatus.
2. Description of Related Art
A handwriting apparatus is generally an assembly of an electromagnetic board and an electromagnetic pen. The conventional handwriting tool may be categorized based on the related stylus, e.g. electromagnetic pen, which may have built-in battery and the other type of electromagnetic pen without inside battery set. The battery-free electromagnetic pen adapts a passive charging technology being disclosed in various prior references such as U.S. Pat. No. 4,848,496, U.S. Pat. No. 5,600,105, and EP0513559 A2.
In general, the battery-free electromagnetic pen takes power from the electromagnetic board since it has no inside battery set. The antenna coil inside the electromagnetic board is employed to deliver the electromagnetic signals/energy to the electromagnetic pen and receive electromagnetic signal there-from. Therefore the behavior of the electromagnetic pen can be detected through the signal pulses with specific range of frequency made by the antenna coil; wherein the signal pulses are such as electromagnetic energy. The electromagnetic pen may store little energy after resonance while it is disposed with a resonance circuit having coils connected with a capacitor.
The electromagnetic board stops emitting signals and becomes a receiver mode after the detection is performed. In the meantime, the battery-free electromagnetic pen delivers the electromagnetic signals made by this pen's oscillation frequency. The oscillation frequency enables the electromagnetic pen consume the stored energy immediately. For this reason, the battery-free electromagnetic pen may not constantly emit electromagnetic signals to the electromagnetic board for a long time. When the energy of electromagnetic pen is exhausted, the electromagnetic board becomes a transmitter mode for emitting the electromagnet energy. After that, the board again changes to be the receiver mode for receiving the signals made by the electromagnetic pen. The operations mentioned above are cyclkally repeated.
Inside the electromagnetic pen, a frequency changer driven by stressing the pen tip is installed. The electromagnetic board includes a frequency counter, analog-to-digital converter, and a control module or a micro-processor. The frequency counter is connected to a phase discriminator or an integrator. The output of integrator is connected to the input of the analog-to-digital converter. That means the change of oscillation frequency of the electromagnetic pen drives the output of integrator to be changed when the user stresses the tip of electromagnetic pen. The output of integrator undergoing the analog-to-digital converter is converted to digital values that sent to the control module. The control module calculates the output coordinates and the digital value indicative of stress of pen tip. The coordinates and the gradation of the pressure are then transmitted to a display device.
When the prior technologies employ the phase discriminator, integrator, or any other method of integration to obtain the pressure-integration signals. The pressure-integration signals are kind of analog signals which may result in ambient interference to the frequency counter. The interference may cause larger voltage error of the output of integrator that seriously affects the gradation of pressure generated by the pen tip.